The website of the team who research palaeoenvironmental change at the Open University

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Isotope geochemistry

Researchers in the Open University's team investigating palaeoenvironmental change use a range of isotope systems to unravel past environmental conditions and processes, and the timescales over which Earth processes took place.

Variations in isotope ratios are generated in the environment in two ways: firstly, isotopes may be fractionated during geological and biogeochemical processes leading to an uneven partitioning between reservoirs, and secondly, the radioactive decay of a parent isotope leads to the growth of a radiogenic daughter isotope. By studying the isotopic compositions of key elements recorded in sedimentary deposits, we can obtain a wide range of information about the past environmental conditions on the Earth’s continents and within its oceans, and about the geological processes that caused environmental change to take place.

The periodic table showing highlights of the isotopes we are developing as proxies and using to understand palaeoenvironmental change

The Nd isotope system: reconstructing past ocean circulation patterns

Ocean circulation, both at the present and in the past, is a central factor in the complex interactions between oceans and climate, and has a controlling effect on marine conditions such as seawater oxygenation and productivity.

Because the residence time of Nd in seawater is shorter than the mixing time of the oceans, water masses in the world’s oceans display distinct Nd isotope compositions. These seawater Nd isotope compositions are ultimately derived from continental rocks that provide dissolved chemical species and particles to seawater through weathering and erosion. In particular, the deep waters in each ocean basin possess distinctive Nd isotope compositions that reflect the areas where these waters originate.

Fish teeth record the Nd isotope signature of the bottom water in which they were deposited; thus, by analysing the Nd isotopic composition of teeth preserved in the sediment we are able to identify deep-water sources in the past and thereby build up a picture of how ocean circulation changed over time.

Oxygen concentrations in seawater have changed significantly in the past, affecting marine life and altering the biogeochemical cycling of many elements in the oceans. Research by members of our team over the last 18 years has focussed on several major events within the last 250 million years when marine anoxia became widespread. Importantly, these events often corresponded to periods of rapid climate change and temperature increase. In the current context of anthropogenic global warming, it is important to understand not only what triggered those past changes but also to put together a history of past changes in seawater oxygenation in order to gain a better idea of how ocean oxygenation may vary in the future. (See also ocean deoxygenation.)

We study past seawater oxygenation using a range of elemental and isotopic proxies, in particular, we work with the molybdenum, rhenium and uranium isotope systems. This facility arises because the isotopic compositions of these elements in marine sedimentary deposits depends on the global oxygenation level of the ocean at the time when the sediment was deposited. Moreover, as Mo, Re and U have different residence times in the ocean and their isotope fractionation displays different sensitivities to marine oxygen concentration, each element will reflect different redox conditions on various timescales. Therefore, combining the information from these three isotope systems allows us to estimate the extent and severity of geological episodes of seawater oxygen changes, and to correlate them accurately with the causative Earth processes and the consequent environmental impacts.

One of the keys to understanding past environmental change is to constrain its timing precisely; Re-Os geochronology is a powerful tool in this regard. Rhenium and osmium are both significantly enriched in the organic-rich sediments that accumulate in seawater under relatively anoxic conditions. One isotope of Re (187Re) is radioactive and decays to 187Os with a half life of approximately ten times the age of the Earth, thus the proportion of 187Os in the marine mudrocks will increase as a function of time and of the Re/Os ratio. Therefore, the analysis of the Os isotope composition of mudrocks can potentially provide an estimate of their absolute age of deposition.

Port Mulgrave, Yorkshire, UK where we collected samples and showed that there large increase in the rate of chemical weathering during the Early Jurassic using the Os isotope system (see Cohen et al. 2004).

Additionally, the osmium isotopic composition of seawater reflects the balance between radiogenic Os (enriched in 187Os) from weathered continental crust, and unradiogenic Os from juvenile basaltic rocks and extraterrestrial sources. Therefore, the seawater Os isotope composition preserved in marine sediments of different age can reflect variations in continental weathering over time.

Unlike more widely used weathering proxies such as 87Sr/86Sr, the marine-residence time of osmium is sufficiently short such that it can resolve glacial-interglacial fluctuations and discriminate between relatively high frequency climatic and relatively low frequency tectonic forcing.

The lithium isotope system shows major fractionation during the weathering of continental silicate minerals, because the light isotope (6Li) is preferentially retained in clay minerals. The weathered Li is enriched in the heavy isotope (7Li) and is carried to the ocean by rivers, driving the seawater isotopic composition to a heavy value that depends on clay production on the continents.

The Li isotopic composition of seawater is then recorded in the tests (microscopic shelly remains) of foraminifera which are eventually preserved in marine sediments. Unlike Sr or Os isotope compositions, the Li isotope composition of riverine fluxes depends only on the intensity of silicate weathering on the continents and should not be affected by the nature of the weathered material. This key distinction between the different isotopic systems means that records of seawater Li isotopic composition in the past may provide complementary information about continental silicate weathering and a new understanding of the carbon cycle in Earth’s history.

Our work on Li isotopes is relatively new, here are some of our published abstracts:

Anand Pallavi, James, R.H. and Mokadem F. (2010). Records of continental weathering for the past 15 Ma from paired analysis of lithium and neodymium isotopes in planktonic foraminifera. 10th International Conference on Paleoceanography.

John, E. H., Anand Pallavi and James, R.H. (2010). Changes in weathering processes in the Oligocene - Miocene: interpretations of Li/Ca, d7Li and eNd data from the Ceara Rise, western equatorial Atlantic. 10th International Conference on Paleoceanography.